Method of enhancing the stability of electroactive polymers and redox active materials

ABSTRACT

This invention relates to a method of enhancing the stability of electroactive polymers, redox active materials, or a composite comprising an electroactive polymer and a redox active material, which comprises depositing on the electroactive polymers, the redox active materials, or the composite, a fluoropolymer by radio frequency sputtering. The invention also relates to electroactive polymers, redox active materials, or a composite comprising an electroactive polymer and a redox active material, that bears a radio frequency sputtered fluoropolymer coating.

FIELD OF THE INVENTION

This invention relates to a method of enhancing the stability ofelectroactive polymers and redox active materials by deposition of afluoropolymer coating by radio frequency sputtering.

DESCRIPTION OF THE RELATED ART

Stability Enhancement of Electroactive Polymers

Electroactive polymers have undergone an unexpectedly rapid developmentin the last half-century. Polyaniline (PANI) is an extensively studiedelectroactive polymer due to its interesting properties and potentialapplications. PANI doped with mobile inorganic anions readily undergoesundoping when immersed in water, and in basic solutions the undopingprocess is particularly rapid. This limits the scope of possibleapplications for these electroactive materials. Some investigations havetherefore been devoted to improve the electrical stability of PANI inaqueous media through either physical or chemical methods.

(1) Modification of PANI Films or Fibers.

Surface graft copolymerization of PANI with a hydrophobic monomer is apractical way to retard deprotonation, and the associated loss ofconductivity. Hydrophobic monomers such as pentafluorostyrene (PFS) andstyrene can be graft copolymerized, in a multi-step method, on thesurface of PANI film. The grafted film exhibits significantly higherstability in maintaining its conductive state than the unqrafted film,either in deionized water or in aqueous media with pH from 2 to 10. Theprotective effect is diminished when the pH exceeds 10. (Zhao Baozong,Neoh K. G., Liu F. T., Kang E. T., Langmuir, 15, 8259 (1999))

U.S. Pat. No. 4,692,225 discloses a method of stabilizing electroactivepolymers to hostile environments. The electroactive polymer may be, forexample, polypyrrole or polyaniline, as a powder, a free-standing filmor preferably in the form of a composite with a substrate, such as afiberglass fabric, impregnated with the electroactive polymer. Themethod comprises encapsulating the electroactive polymer with a suitableepoxy, polyimide or bis-maleimide resin, as an encapsulating agent. Thepreferred method involves “coating or pre-pregging” the fabric of theelectroactive polymer with an epoxy resin and curing the resultingsystem.

U.S. Pat. No. 5,393,610 and U.S. Pat. No. 5,198,266 disclose a methodfor stabilizing the shelf life conductivity of conductive coating suchas polypyrrole or nickel sulfide on a substrate such as fiberglass byincorporating a polyphenol or a polysiloxane in the conductive coating.The polyphenol is derived from a phenolic material in the form of aphenol-formaldehyde monomer or an oligomer of phenol-formaldehyde, andthe polysiloxane is derived from a siloxane. Upon heating, the phenolicmaterial cures to a polyphenol and the siloxane is converted to apolysiloxane, forming the stabilizing material on the conductivepolypyrrole or nickel sulfide coated substrate. The phenolic material ispreferably incorporated directly into the solution formulation forpreparing the conductive nickel sulfide or polypyrrole on the substrate,and the polyphenol stabilizer is formed together with the conductivecoating on the substrate by heating. In the case of the silane orsiloxane material, the conductive polypyrrole or nickel sulfidepreferably is formed first on the substrate, and the conductive coatedsubstrate is contacted with a solution of the siloxane followed byheating to convert the siloxane to a polysiloxane protectiveovercoating.

Japan Patent JP 2221851 discloses a method of improving theelectrochemical properties of conducting polymer chemical sensor bycoating the polymer with a hydrophobic polymer via spin coating or bythe use of adhesives.

European Patent EP 0783050 describes the stability enhancement ofelectroactive polymers. A method of depositing a electroactive polymerfilm on a textile fabric substrate is provided by the oxidativepolymerization of a pyrrole compound in the presence of a dopant anionand a stabilizing agent having the formula:

wherein R1, R2, R3 and R4 are independently selected from H, OH, and OR,and R is C1-C8 alkyl; and R5 and R6 are independently selected from H,COOH and SO₃H.

These methods achieve the stability enhancement of electroactivepolymers by either surface or bulk modification. However, in the surfacemodification techniques mentioned above, the surface conductivity of afilm of the electroactive polymer will decrease after the coating orgrafting process, and the thickness of the coating or graft layer isdifficult to control. On the other hand, the bulk modification of theelectroactive polymers to form a composite will change the nature of theelectroactive polymer, resulting in a lower conductivity. Hence thesemethods are useful only for special products and for certainapplications.

(2) Using Organic or Polymeric Anions as Dopants

An alternative way of retarding the deprotonation of the conductingpolymer is to minimize the loss of counterions from the electroactivepolymer. Loss of counterions is accompanied by deprotonation of theelectroactive polymer, which then reverts to a non-conductive state.PANI salt films cast from salt solutions retain anions better whentreated with water if larger organic anions (e.g. sulfosalicylic acid)rather than smaller inorganic acid anions (e.g. Cl⁻, ClO₄ ⁻) are used.(Neoh K. G.; Kang E. T.; Tan K. L. Polym. Degrad. Stabil. 43, 141(1994)) Polymeric anions incorporated into the electroactive polymermatrix are not easily lost when the polymer is immersed in water.However, these methods cause some problems. Films doped with largeorganic or polymeric anions have substantially weaker mechanicalproperties than films doped with inorganic acid anions, as well as lowerdoping levels and conductivity. Doped PANI films prepared by blendingPANI with poly(acrylic acid) in N-methyl-2-pyrrolidinone (NMP) have theproblem of non-uniformity and inefficient doping (Chen, S. A.; Lee, H.T. Macromolecules, 28, 2858(1995)), and the synthesis of PANI inpolymeric acids leads largely to soluble product (Neoh K. G.; Kang E.T.; Tan K. L. J. Phys. Chem. B. 101, 726(1997)).

(3) Self-Doped Polymers

In these polymers, functional acid groups such as —SO₃H covalentlyattached on the polymer chains of the electroactive polymer serve ascounterions in the polymer matrix. These counterion groups can beintroduced by treatment of PANI or polypyrrole (PPY) with fumingsulfuric acid (Yue J.; Wang Z. H.; Cromack, K. R.; Epstein A. J.;MacDiarmid, A. G. J. Am. Chem. Soc. 113, 2665(1991); Kang E. T.; Neoh K.G.; Woo Y. L.; Tan K. L. Polym. Commun. 32, 412 (1991)) or withchlorosulfonic acid (Murata K.; Teshima S.; Aizawa R.; Asako Y.;Takahashi K.; Hoffman B. M. Synth. Met. 96, 161 (1998)) The substitutinggroups diminish the conjugation of polymer units, resulting in theself-doped polymer being less conductive (about one or more order ofmagnitude in loss of conductivity) than the externally doped polymer.The self-doped polymers usually are of poor mechanical strength. It hasbeen verified that the immobilized —SO₃ ⁻ groups in the self-dopedpolymer are stable in water.

It should be noted that the undoping process would be accelerated inbasic solutions. Electroactive polymers modified by using organic orpolymeric anions as dopants or self-doped polymers as mentioned aboverapidly revert to the insulating state especially in basic solution withhigh pH value.

Stability Enhancement of Redox Active Materials

An example of redox active materials includes1,1′-disubstituted-4,4′-bipyridinium salts, which are commonly known asviologens. Viologens exist in three redox states, as dication, radicalcation, and di-reduced species. Almost all of the applications ofviologens are related to electron transfer reaction between the firsttwo redox states. Viologens display a photochromic effect caused byreduction of the viologen dication to its radical cation and this hasreceived considerable attention for application to electrochromicdisplays, dosimeters and actinometers. However, the radical cations ofviologens are not stable in air because of the highlyoxidation-sensitive nature of the radical cation. Reaction withmolecular oxygen is particularly fast. In order to shield viologens fromoxygen or other oxidizing agents, different methods have been attemptedto maintain the stability of the radical cations.

(1) Vermeulen [L. A., Thompson M. E. NATURE, 358, 656 (1992)] describesstable photo-induced charge separation in layered viologen compounds. Itis reported that the photochromic behavior of novel layered zirconiumphosphonate/viologen compounds shows very efficient photo-induced chargetransfer. The compounds form a charge-separated state that is long-livedand stable in air. Spectroscopic studies indicate that the photoproductis a dialkyl viologen radical cation, produced in the interlamellarregion of the zirconium phosphonate. It is suggested that the remarkablestability of the charge-separated state arises from structural featuresthat allow for stabilization of the radicals by delocalization andshielding from molecular oxygen.

(2) U.S. Pat. No. 5,516,462 describes an enhanced cycle lifetimeelectrochromic system. In this electrochromic systems, the increase incycle lifetime is a product of one or more of the following: novel orknown asymmetric viologen compounds, mixed electrolyte systems, andmixed solvent systems.

(3) European Patent EP 0682284 describes an electrochromic device havingexcellent coloring properties which uses an electrolyte solutioncomposed of a viologen and a specific alcohol solvent as one componentof the device, or uses an electrolyte film produced by immersing orimmobilizing the solution into a porous film as one component of thedevice. It further provides an electrochromic device having improvedoperation life and stability to operate at a lower temperature andcomprising an electrolyte placed between a pair of electrodes, whereinthe electrolyte is in the form of an electrolyte solution prepared bydissolving a viologen derivative in an organic solvent or wherein theelectrolyte is in the form of an electrolyte film prepared by immersingand immobilizing the solution into a porous film.

In both U.S. Pat. No. 5,516,462 and EP patent 0682284 viologens areemployed as the electrolyte in electrochromic devices. In order to avoidoxidation, the redox active viologen is usually tightly sandwichedbetween two electrodes and/or support substrates like glass.

BRIEF SUMMARY OF THE INVENTION

Surprisingly, a different approach has been found for stabilizing thedesired conductive properties or oxidation states of electroactivepolymers or redox active materials. In this process, electroactivepolymers or redox active materials are coated with a fluoropolymer,using radio frequency (RF) sputtering.

In one aspect, the invention provides a method of enhancing thestability of an electroactive polymer, a redox active material, or acomposite comprising an electroactive polymer and a redox activematerial, which comprises depositing on the electroactive polymer, theredox active material, or the composite, a fluoropolymer coating byradio frequency sputtering.

In another aspect, the invention provides an electroactive polymer, aredox active material, or a composite comprising an electroactivepolymer and a redox active material, that bears a radio frequencysputtered fluoropolymer coating.

The present method employs a thin, fluoropolymer coating deposited onthe electroactive polymer or redox active material via RF sputtering.The fluoropolymer coating can be transparent. No adhesives or epoxyresin are needed to apply the fluoropolymer coating, and no solvent isrequired in the coating process, and the surfaces of the electroactivepolymer or redox active material to be coated do not requirepre-treatment. The use of RF sputtering in the present invention leadsto a thickness of fluoropolymer coating that can be controlled bychanging the sputtering time at fixed power levels, or by carrying outthe sputtering for a certain time at different radio frequency powers.The thickness of the fluoropolymer may be, for example, from about 10 nmto about 100 nm (e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100nm).

The present invention does not result in significant changes to the bulkproperties of the electroactive polymer or redox active material. Thecoating process can be carried out after the electroactive polymer orredox active materials have attained the desired level of activity.Alternatively, further photoinduced reactions can be carried out in theelectroactive polymers or redox active materials after the coatingprocess, in the embodiments where the fluoropolymer coating has a thinand transparent nature.

The present invention relates to a method of enhancing the stability ofelectroactive polymers and redox active materials. In the case ofelectroactive polymers, for example polyaniline, the fluoropolymercoating enhances the electrical stability of the polymer, especiallywhen it is immersed in aqueous media. In the case of redox activematerials, for example viologens, the fluoropolymer coating enhances theoxidation resistance (also referred to as photochromic stability) of theviologen. The preferred method of enhancing the stability of theelectroactive polymer or redox active material involves the depositionof a layer of a fluoropolymer, for example fluorinated ethylenepropylene copolymer (FEP), having a thickness of about 10 nm or more onthe surface of the electroactive material. When a polyaniline filmsputtered with fluoropolymer is immersed in aqueous media such as water,or in a basic solution, degradation of the electrical conductivity ofthe film is retarded. Similarly, when a viologen film sputtered withfluoropolymer is excited under UV irradiation to the radical cationstate, a resulting coloration of the film is maintained for a longerperiod of time due to retardation of the reaction of the viologen filmwith an oxidative atmosphere, such as an atmosphere containing oxygen.

The stability enhancement of both the electroactive polymer and theredox active material is evident from conductivity measurement and/orspectroscopic analysis.

DESCRIPTION OF THE FIGURES

FIG. 1 displays the relationship between the sheet resistance and thewater treatment time for a doped polyaniline coating coated with afluorinated ethylene propylene copolymer (Example 1), a dopedpolyaniline free-standing film coated with a fluorinated ethylenepropylene (Example 3), a polyaniline-viologen thin film assembly coatedwith a fluorinated ethylene propylene after UV irradiation (Example 5)and a polyaniline-viologen thin film assembly irradiated with UV aftercoating with a fluorinated ethylene propylene (Example 6), when immersedin water.

FIG. 2 displays a UV-visible absorption spectrum of a non-treatedpolyaniline coating before immersion in water, of a non-treatedpolyaniline coating after a 5 minute immersion in water, and of atreated (fluorinated ethylene propylene copolymer coating) polyanilinecoating after a 3 hour immersion in water.

FIG. 3 displays a UV absorption spectrum of a non-fluoropolymer coatedvinyl benzyl viologen grafted film, in the presence of oxygen, 1 minuteafter the formation of the viologen radical cation, and a sequence of UVabsorption spectra of a fluoropolymer coated vinyl benzyl viologengrafted film, in the presence of oxygen, 0, 2, 4, and 6 minutes afterthe formation of the viologen radical cation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The electroactive polymer can be selected, for example, frompolyanilines [PANI], polypyrroles, polythiophenes, polyacetylenes[(CH)x], poly(para-phenylene)[PPP] and poly(p-phenylene vinylene)[PPV].The electroactive polymers can be either unsubstituted or substituted.Examples of substituted polyanilines include, for example,poly(o-toluidine), and halogenated polyanilines such aspoly(2-fluoroaniline), poly(o-chloroaniline), and the like. Polyanilineis commercially available. Examples of substituted polypyrroles include,for example, N-substituted pyrroles such as 1-(carboxyalkyl) pyrrolesand 1-(4-nitrophenyl) pyrrole, and the like. Examples of substitutedpolythiophenes include, for example, poly(3,4-ethylenedioxythiophene),poly[3-(4-octylphenyl)thiophene], and the like.

The unsubstituted or substituted electroactive polymers can also containlarge organic or polymeric anions as dopants, for example as describedby Neoh et al. in Polym. Degrad. Stabil. 43, 141 (1994), or they can beself-doped polymers, for example as described by Yue et al. in J. Am.Chem. Soc. 113, 2665(1991), by Kang et al in Polym. Commun. 32, 412(1991), or by Murata et al. in Synth. Met. 96, 161 (1998).

An electroactive polymer may be in any form, for example in the form ofa free-standing film, a thin film coating on inert substrates such asLDPE (low density polyethylene) film, or a coating on viologen-graftedLDPE films. One method of preparing the electroactive polymer on asubstrate involves placing the substrate in a polymerizing mixture ofthe appropriate monomer to obtain a thin film coating of theelectroactive polymer. Other preferred methods for preparingelectroactive polymers involve casting the electroactive polymer from asolution into a thin film, or synthesizing the electroactive polymer infilm form via electrochemical means (Tan, H. H.; Neoh, K. G.; Liu, F.T.; Kocherginsky, N.; Kang, E. T. J. Appl. Polym. Sci., 80, 1, (2001);Cen, L.; Neoh, K. G.; Kang, E. T. Langmuir, 18, 8633, (2002)). Theelectroactive polymer can have: 1) only reduced repeating units (termedleucoemeraldine in the case of polyaniline), 2) only oxidized repeatingunits (termed pernigraniline in the case of polyaniline), or 3) varyingratios or both species (termed emeraldine in the case of polyanilinewith a 1:1 ratio of reduced and oxidized repeating units). Theelectroactive polymer can exist in the form of its base or itsprotonated form.

The redox active material can be selected, for example, from viologenssuch as described in European Patent No. 0 682 284 and U.S. Pat. No.5,516,462, the relevant parts of which are hereby incorporated byreference. Mention is made of benzyl viologen and vinylbenzyl viologenand their salts, particularly the dihalide salts. Viologens have beenstudied extensively, and further examples of suitable viologens can befound in the literature (for example P. M. S. Monk, The Viologens:Physicochemical Properties, Synthesis and Applications of the Salts of4,4′-Bipyridine, John Wiley & Sons, Chichester, 1998, which is herebyincorporated by reference). The redox active material may be in the formof a film or it can be immobilized on the surface of a substrate (Zhao,L. P.; Neoh, K. G.; Kang, E. T. Chemistry of Materials, 14, 1098,(2002);Liu, X.; Neoh, K. G.; Zhao, L. P.; Kang, E. T. Langmuir, 18, 2914,(2002)).

The invention also contemplates a composite or a material that comprisesboth an electroactive polymer and a redox active material. A compositethat comprises both an electroactive polymer and a redox activematerial, such as a viologen, can exhibit interesting changes inconductivity under UV irradiation (Ng, S. W. ; Neoh, K. G.; Sampanthar,J. T. ; Kang, E. T.; Tan, K. L. J. Phys. Chem. B, 105, 5618, (2001)).Hence, irradiation of selected areas can provide conductivity toselected areas, and this has pertinence, for example, for patternfabrication and electronic design. In some embodiments, the thin andtransparent nature of the fluoropolymer coating is useful in thesesystems that comprise both an electroactive polymer and a redox activematerial, as the changes in conductivity obtained by UV irradiation canbe carried out after the fluoropolymer coating is applied.

As fluoropolymer to be deposited, there is preferably used aperfluorinated polymer. Example of perfluorinated polymers includefluorinated ethylene propylene copolymers (FEP). Such copolymers arecommercially available, for example, under the trademarks Hostaflon FEPand Teflon FEP. Examples also include polytetrafluoroethylene, which iscommercially available, for example, under the trademarks Fluon,Hostaflon TF and Teflon TFE. These are commercially available fromGoodfellow Cambridge Limited, and from other sources. Fluorinatedethylene propylene copolymers have excellent chemical resistance andgood weathering resistance, along with good radiation resistance andimpact strength. Though not quite crystal clear, the copolymer istransparent (from about 250/300 nm to about 7000 nm) and it is colorlessor has a slight bluish tinge.

In the present invention, a layer of fluoropolymer is sputtered on thesurface of an electroactive polymer or redox active material. Onesputtering method involves the placement of the fluoropolymer as atarget in a radio frequency (RF) magnetron sputter gun powered by a RFgenerator in a sputtering chamber. A sample to be coated (i.e.electroactive polymer or redox active material) is fixed on a rotarysample stage placed at a suitable distance away from the target. The RFsputtering is carried out in a reduced pressure environment, for exampleat a pressure of 12 mTorr, and a sputtering gas, for example argon, isintroduced into the chamber to form the plasma for bombarding the target[Zhang Yan, Yang G. H., Kang, E. T. and Neoh, K. G. Langmuir, 2002, 18,6373]. For a specific fluoropolymer, the thickness of the fluoropolymerlayer is mostly dependent on a) the distance between the electroactivematerial sample and the fluoropolymer target, b) the power of the radiofrequency generator, and c) the length of sputtering time, and theseconditions can be independently varied to obtain the wanted thickness offluoropolymer. A suitable distance between the fluoropolymer target andthe sample is about 13 cm, and the RF power can be in the range from 100W to 300 W, for example 150 W, although other suitable distances or RFpower values can be readily determined. For some applications, thesputtering time can be 100 seconds or more, preferably from 100 s to 600s, to obtain the desired thickness of fluoropolymer coating. Normally,the distance between the fluoropolymer target and the sample to becoated is fixed for a specific RF magnetron sputtering instrument,therefore the thickness of the fluoropolymer coating is normally variedby changing the sputtering time and the RF power. The thickness of thefluoropolymer coating is also dependent on the nature of thefluoropolymer, as different fluoropolymers are deposited at differentrates. A person skilled in the art will be able to determine whichthickness of fluoropolymer is suitable for enhancing the stability ofthe electroactive polymer or the redox active material. For somepurposes, a sputtered fluoropolymer layer thickness from about 10 nm toabout 100 nm or more is preferred, and a thickness from about 10 toabout 50 nm is more preferred.

In one embodiment, with a fluorinated ethylene propylene copolymer (FEP)target, with a target and sample (i.e., electroactive polymer or redoxactive material) separation of 13 cm and with a RF power of 150 W, thethickness of the FEP deposited on the sample can be varied from lessthan 10 nm for a 100 seconds sputtering time, 16 nm for 300 seconds, and43 nm for 600 seconds.

A preferred application and good results are obtained when thefluoropolymer layer is transparent and of thickness in the nanometerscale. Such a coating is adherent and allows further photo-inducedreaction in the active materials to be carried out after the coatingprocess. The sputtering technique can be applied to fluoropolymers ingeneral. After the completion of the sputtering process, the stabilityof the electroactive polymer or redox active material is enhancedwithout further processing. The thickness of the sputtered fluoropolymercoating can be determined by using, for example, a surface profilinginstrument such as an Alpha-STEP 500 Surface Profiler (KLA-Tencor Co.San Jose, Calif., USA).

Applications

The present invention may further extend the applications ofelectroactive polymers and redox active materials such as polyaniline.The sputtered fluoropolymer coating may work as a barrier to prevent thedegradation of conducting polymers when they are used as coatings inaqueous environments. This technology may also be employed inelectrochemical devices. In addition, this invention may also findapplications in maintaining the electrical stability of conductingpolymer patterns formed on semiconductors in oxidative or basicatmospheres, using the sputtered fluoropolymer as the passivation layer.In the area of optical and radiation technology, the use of sputteredfluoropolymer to prolong the photochromic effect may have potentialapplications in electrochromic displays, smart windows, dosimeters andactinometers.

EXAMPLES

The following specific examples are provided to illustrate thisinvention and the manner in which it may be carried out. It will beunderstood, however, that the specific details given in each examplehave been selected for the purpose of illustration, and are not to beconstrued as being limitations on the scope of the invention.

In all the examples there was used a magnetron with a RF power of 150 Wand distance between a fluoropolymer target and a sample to be coated ofabout 13 cm.

In the examples, the fluorinated ethylene propylene copolymer used is asobtained from Goodfellow Cambridge Limited, under the trademarks TeflonFEP or Hostaflon FEP, and the polytetrafluoroethylene used is asobtained under the trademarks Fluon, Teflon TFE or Hostaflon TF.

Example 1

A coating of polyaniline (in the emeraldine state) on a low-densitypolyethylene (LDPE) substrate was prepared according to a previousreport (Ng S. W. , Neoh K. G., Sampanthar J. T. , Kang E. T., Tan K. L.J. Phys. Chem. B, 105, 5618(2001), the disclosure of which is herebyincorporated by reference).

Before the sputtering process, the polyaniline coated LDPE film wasredoped to the conducting state (sheet resistance˜2.4×10³Ω/sq) bytreatment with 0.5M H₂SO₄. This film was then sputtered with fluorinatedethylene propylene copolymer (FEP) for 100 s using argon, to deposit FEPcopolymer on the polyaniline coated LDPE film. A fluoropolymer coatingof about 10 nm was obtained.

When the film with the FEP layer was immersed in water, the sheetresistance (Rs) was initially 9×10³Ω/sq, and it increased to 7.4×10⁵Ω/sqafter 3 hours. The change in Rs with time of treatment in water is shownin FIG. 1. In comparison, the sheet resistance of a similar polyanilinefilm without FEP coating would increase to more than 6.6×10⁹ Ω/sq afteronly 5 min immersion in water.

The UV-visible absorption spectrum of polyaniline is indicative of itsdoped state. In FIG. 2, the UV-visible absorption spectra of thepolyaniline films with and without the FEP coating after immersion inwater are compared with that of doped polyaniline before treatment withwater. In the spectrum of the doped PANI film before water treatment,there is no absorption band at around 620 nm, while a band at 430 nm anda high absorption tail beyond 800 nm exist. These two bands have beenassigned to excitations from the highest and second highest occupiedenergy bands to the partially filled polaron band in PANI, indicatingthe polymer is in its conductive state. In the case of the film with FEPcoating after 3 h immersion in water, the spectrum is similar to that ofthe doped polyaniline prior to treatment with water. Without a FEPcoating, the doped PANI film undopes very quickly upon exposure towater. The intensity of the 620 nm band increases while the bands at 430nm and beyond 800 nm decrease, implying a conversion of theelectroactive polymer to the non-conductive (undoped) state. Theabsorption peaks found at 325 and 625 nm after 5 minutes immersion inwater are characteristic of undoped polyaniline (emeraldine base). Thusthe UV-visible absorption spectroscopy results are consistent with thesheet resistance measurements which show that with the FEP coating thepolyaniline remains in the conductive state even after 3 h in water.

Example 2

A polyaniline coated LDPE film was synthesized as described inExample 1. The film was then sputtered with a fluorinated ethylenepropylene copolymer (FEP) for 600 s under an argon atmosphere. The filmwith the FEP layer was then immersed in 0.01 M NaOH. Sheet resistanceincreased gradually from an initial value of 5.06×10⁴Ω/sq to a value of6.5×10⁶Ω/sq after 3 hours. On the other hand, the polyaniline filmwithout FEP coating was undoped immediately after immersion in 0.01 MNaOH solution and was changed to an insulating state.

Example 3

A polyaniline free-standing base film (in the emeraldine or 50%oxidation state) was synthesized via the oxidative polymerization ofaniline using ammonium persulfate in 0.5M H₂SO₄ (MacDiarmid A. G. et al.Synth. Met. 18, 285 (1987), the disclosure of which is herebyincorporated by reference), in accordance with the following procedure.Polyaniline powder was undoped using excess 0.5 M NaOH and thepolyaniline (emeraldine) base powder was dissolved in N-methylpyrrolidinone (NMP), to give a solution of 8 wt %. A free-standing filmof 10-20 μm thickness was cast from the polyaniline base in the NMPsolution. The polyaniline free-standing film was then doped by treatmentwith 0.5M H₂SO₄ and dried under reduced pressure. The film was thensputtered with fluorinated ethylene propylene copolymer (FEP) for 100 sunder an argon atmosphere to obtain a fluoropolymer coating of about 10nm. The film with the FEP layer was immersed in water. Sheet resistance(Rs) increased slowly from an initial value of 3.7×10³Ω/sq to a value of3.6×10⁴Ω/sq after 3 hours. The change in Rs with the time of treatmentin water is shown in FIG. 1. In contrast, the sheet resistance of thedoped free-standing PANI film without FEP coating increased from1.2×10³Ω/sq to 3.3×10⁵Ω/sq after 3 h.

Example 4

A polyaniline coated LDPE film was synthesized as described inExample 1. The film was then sputtered with polytetrafluoroethylene(PTFE) for 100 s under an argon atmosphere. The film with the PTFE layerwas immersed in water. Sheet resistance increased from 8.5×10⁴ to4.2×10⁶ Ω/sq after the first 15 min, and then slowly increased to morethan 6.6×10⁹Ω/sq after 2 hours. While the PTFE coating offers a degreeof protection to the polyaniline from the aqueous environment, it is notas effective as a FEP coating formed under the same sputteringconditions (RF power and sputtering time) possibly due to a lowerdeposition rate of the PTFE polymer.

Example 5

A polyaniline-benzyl viologen chloride thin film assembly on LDPE wassynthesized according to a previously published method. (Zhao Luping,Neoh K. G., Kang E. T., Chem. Mater. 14, 1098(2002), the disclosure ofwhich is hereby incorporated by reference). The polyaniline-viologenfilm was converted to a conductive state via exposure to nearUV-irradiation from a 1 kW mercury lamp for 1 h [please refer to (Ng, S.W. ; Neoh, K. G.; Sampanthar, J. T. ; Kang, E. T.; Tan, K. L. J. Phys.Chem. B, 105, 5618, (2001))]. It was then sputtered with fluorinatedethylene propylene copolymer (FEP) for 100 s, using argon, to obtain afluoropolymer coating of about 10 nm. The film with the nanostructuredFEP layer was immersed in water. Sheet resistance (Rs) increased slowlyfrom an initial value of 4.4×10⁴Ω/sq to a value of 2.3×10⁶Ω/sq after 3hours. The change in Rs with time of treatment in water is shown inFIG. 1. In comparison, the sheet resistance of a similarpolyaniline-viologen film without FEP coating would increase to morethan 6.6×10 ⁹Ω/sq after only 5 min immersion in water.

Example 6

A polyaniline-benzyl viologen chloride film assembly was synthesized asdescribed in Example 5. Before exposure to UV irradiation, the film wassputtered with fluorinated ethylene propylene copolymer (FEP) for 100 s,using argon. The film was then exposed to near UV-irradiation emittedfrom a 1 kW mercury lamp for 1 h to render the film electricallyconductive. The film was immersed in water and its sheet resistance (Rs)was observed to increase slowly from an initial value of 3.6×10⁴Ω/sq toa value of 1.1×10⁷ Ω/sq after 3 hours. The change in Rs with time oftreatment in water is shown in FIG. 1. In comparison, the sheetresistance of a similar polyaniline-viologen film without FEP coatingincreases to more than 6.6×10⁹Ω/sq after only 5 minutes of immersion inwater.

Example 7

Vinylbenzyl viologen dichloride grafted on a LDPE film was synthesizedaccording to a previously published method. (Liu Xin, Neoh K. G., ZhaoLuping, and Kang E. T., Langmuir, 18, 2914 (2002), the disclosure ofwhich is hereby incorporated by reference). This viologen-grafted filmwas sputtered with fluorinated ethylene propylene copolymer (FEP) for100 s, using argon, to obtain a fluoropolymer coating of about 10 nm.The FEP coated viologen film was then exposed to near UV-irradiation for10 min in order to form viologen radical cations. The formation of theviologen radical cations resulted in a blue coloration, and UV-visibleadsorption spectroscopy showed an absorption band at about 613 nm. Upontermination of the irradiation, with oxygen present, the viologenradical cations rapidly react with oxygen to return to the dicationstate and the color bleaches. In the absence of the FEP coating, theblue coloration is lost within 1 minute. With the FEP coating,availability of oxygen is restricted and the bleaching process is thusretarded as shown in FIG. 3.

To those skilled in the art to which this invention relates, manychanges in construction and widely different embodiments andapplications of the invention will suggest themselves without departingfrom the spirit and scope of the invention. The disclosures anddescription therein are purely illustrative and are not intended to bein any sense limiting.

The documents cited and referred to above are hereby incorporated byreference.

1. A redox active material, or a composite comprising an electroactivepolymer and a redox active material, that bears a coating having athickness of from about 10 nm to about 40 nm, the coating consisting ofa radio frequency sputtered fluoropolymer.
 2. A method for preparing aredox active material or a composite comprising an electroactive polymerand a redox active material, according to claim 1, which methodcomprises depositing on the redox active material, or the composite, afluoropolymer coating by radio frequency sputtering, the fluoropolymercoating having a thickness of about 10 nm to about 40 nm.
 3. A methodaccording to claim 2, wherein the electroactive polymer is selected fromthe group consisting of unsubstituted or substituted polyaniline,polypyrrole, polythiophene, polyacetylene, poly(para-phenylene),poly(p-phenylene vinylene), and their derivatives.
 4. A method accordingto claim 2, wherein the electroactive polymer is unsubstituted orsubstituted polyaniline.
 5. A method according to claim 2, wherein theredox active material is a viologen.
 6. A method according to claim 2,wherein the redox active material is selected from the group consistingof benzyl viologen and vinyl benzyl viologen.
 7. A method according toclaim 2, wherein the fluoropolymer is a perfluorinated polymer.
 8. Amethod according to claim 2, wherein the fluoropolymer is a fluorinatedethylene propylene copolymer.
 9. A method according to claim 2, whereinthe fluoropolymer is polytetrafluoroethylene.
 10. A method according toclaim 2, wherein the composite comprising an electroactive polymer and aredox active material, is subjected to UV irradiation to render itelectrically conductive before deposition of the fluoropolymer.
 11. Amethod according to claim 2, wherein the composite comprising anelectroactive polymer and a redox active material, is subjected to UVirradiation to render it electrically conductive after deposition of thefluoropolymer.
 12. A method according to claim 2, wherein theradiofrequency sputtering is carried out with a distance of about 13 cmbetween a fluoropolymer target and the redox active material orcomposite, with a radio frequency power of from about 100 W to about 300W, and for a time of from about 100 seconds to about 600 seconds.
 13. Aredox active material according to claim 1 that displays oxidationresistance when contacted with an oxidative atmosphere.
 14. A compositeaccording to claim 1 that displays electrical stability when immersed inan aqueous medium.
 15. A composite according to claim 1, wherein theelectroactive polymer is selected from the group consisting ofunsubstituted or substituted polyaniline, polypyrrole, polythiophene,polyacetylene, poly(para-phenylene), poly(p-phenylene vinylene), andtheir derivatives.
 16. A composite according to claim 1, wherein theelectroactive polymer is unsubstituted or substituted polyaniline.
 17. Aredox active material or composite according to claim 1, wherein theredox active material is a viologen.
 18. A redox active material orcomposite according to claim 1, wherein the redox active material isselected from the group consisting of benzyl viologen and vinyl benzylviologen.
 19. A redox active material or composite according to claim 1,wherein the fluoropolymer is a perfluorinated polymer.
 20. A redoxactive material or composite according to claim 1, wherein thefluoropolymer is a fluorinated ethylene propylene copolymer.
 21. A redoxactive material or composite according to claim 1, wherein thefluoropolymer is polytetrafluoroethylene.